Correct disk's over committed size computing error

There's a error in the disk's over committed size computing for the
instance which using RAW type disk. This issue caused
disk_available_least resource counting increasing.

Fixes bug: 1167695

Change-Id: I4e6cf22a9cf66a757e3641e2a0be0b7699cae69d
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Correct disk's over committed size computing error"